The present invention relates to optical devices and the alignment thereof. More particularly, the present invention relates to a system and method for passively aligning and coupling optical devices with an external mode size transformer.
Optical communications systems offer the potential for widespread delivery of broadband services to businesses and residences if the cost of optical termination can be reduced. An optical communications system can comprise an optical transmitter, a fiber optic transmission line, and an optical receiver. For typical telecommunication applications, the transmitter is comprised of a direct-modulated semiconductor laser diode 100 as illustrated in FIG. 1. The transmission line is single-mode fiber and the receiver is a semiconductor photo-diode (not shown). In recent years the cost of single-mode fiber has dropped dramatically relative to that of metallic transmission media, but the cost of the opto-electronic devices used in the transmitters and receivers are still expensive relative to their electronic counterparts.
In addition to the cost of opto-electronic devices themselves, a time-consuming and costly step in the manufacture of laser transmitter modules involves the alignment of the fiber optic pigtail 110 (the short fiber optic lead that protrudes from the module package for optical interconnection) with the laser diode facet 115 (the point on the edge of the diode from which the laser beam emerges). For optimal transfer of laser energy from the facet 115 to the fiber core 120 (coupling efficiency), the tip 125 of the fiber pigtail 110 typically is precisely positioned with respect to the laser facet 115 prior to fastening and encapsulation in the module package. This is usually done using active alignment techniques, in which the laser diode 100 is powered up and the output optical power is monitored via the pigtail 110 while manipulating the pigtail tip 125 in up to five dimensionsxe2x80x94transverse A, lateral B, longitudinal C, and axial pitch D and yaw E (FIG. 1). This step distinguishes opto-electronic device packaging from semiconductor device packaging, and is considered the prime factor impeding further cost reduction in laser module production. Passive alignment techniques aim to eliminate this active alignment step in the packaging process.
Passive alignment of laser to fiber using purely mechanical means can be complicated by the extremely tight tolerance required to achieve adequate coupling efficiency. The reason for this tight tolerance is because of the small size of the optical mode of the laser (optical mode refers to the spatial distribution of electromagnetic energy of the fundamental mode propagating from the laser waveguided region through the core of the single-mode fiber). Typical semiconductor lasers have mode sizes of approximately 1 micron (xcexcm) and emission angles of more than 30xc2x0 in the transverse dimension; furthermore, the emission beam is an expanding elliptical cone 130 spreading more in the transverse direction than in the lateral direction. The fiber core 120, on the other hand, supports a weakly guided optical mode that is typically 8-10 xcexcm in size and circular in shape with a 6-7xc2x0 acceptance angle. If the fiber pigtail 105 is simply butt-coupled to the laser 100, the modal mismatch results in poor coupling efficiency. Coupling efficiency can be improved by fabricating a conical or spherical lens 135 directly onto the tip of the fiber; the lens alters the mode size and acceptance angle of the fiber 135 so as to better match that of the laser facet 115. However, this increased coupling efficiency occurs at the expense of tighter alignment tolerance. The sub-micron tolerance needed to passively align a tensed fiber is tighter than can be reliably maintained with current automated assembly equipment.
There has been published investigation into the use of mode size transformers to address the current difficulties of passive alignment. A conventional mode size transformer (also called spot-size converter or mode expander) employs a specially tapered waveguide structure that increases the mode size of the laser to match that of the fiber. In doing so, larger alignment tolerances are possible with good coupling efficiencies, offering a way to lower packaging cost. Concurrently, the use of conventional micromachined silicon submount technology (also called silicon optical bench or silicon platform) for mechanical positioning of fibers with respect to other optical or opto-electronic devices is rapidly advancing for passive alignment and automated packaging of integrated optics (integrated optics refers to the combining of multiple optical elements within a single module by waveguiding light between them). A common and conventional silicon submount structure can comprise a V-groove having dimensions suitable for holding a fiber with precise lateral, transverse and axial position limited only by the +/xe2x88x921 xcexcm tolerance of fiber corto-cladding concentricity. Employing a mode size transformer that provides good laser-to-fiber coupling even when the laser and fiber are misaligned up to 1 micron from their ideal alignment, then, would permit the use of silicon submount technology for laser-to-fiber passive alignment This is the goal of the conventional published mode size transformer investigation.
Different conventional approaches have been investigated for monolithically integrating a mode size transformer into the semiconductor layers of a laser diode. A conventional expanded mode laser design can include a laser waveguided (active) region that is laterally tapered and built on top of a uniform weak passive waveguide layer. Adiabatic mode expansion between the active and passive layers of this structure can result in a significant reduction of the transverse emission angle of the laser, thus permitting much greater laser-to-fiber coupling efficiency. Three conventional different integrated mode transformer structures have been evaluated: one with a transversely tapered waveguide, one with a laterally tapered waveguide, and one comprising a small cross section of an active layer. Some conventional structures employ a shape in the laterally tapered active region that has been optimized. However, a disadvantage of the conventional integrated mode size transformer is the greater complexity and cost of processing the laser diode wafer to incorporate these waveguide structures.
A tapered waveguide structure external to the laser diode can also function as a mode size transformer. Tapered polymer waveguides designed to perform mode transformation between the laser facet and the optical fiber have been described in the conventional art. Such structures can include two laterally tapered waveguide layers stacked one on top of the other. One of the waveguide layers can form the input section that is optimized for coupling with the laser facet and confining the fundamental mode; the input section up-tapers laterally to a larger output section that is matched to the mode diameter of the fiber core. The other waveguide layer can laterally taper from zero width to the width of the first layer at the output section; the first layer can support mode expansion in the transverse dimension between the input section and the output section such that the output mode profile is approximately circular (i.e. matched to the fiber mode profile). One advantage of this conventional tapered waveguide structure is its ease of fabrication using polymer materials. However, polymer materials have not been widely accepted in telecommunications applications due to concern over long-term optical stability of polymers. Silica and glassy materials are typically preferred over polymers for use in optical waveguides intended for telecommunications because of their robust environmental stability.
Accordingly, there is a need in the art for a method and system for efficiently coupling one optical device to another. More specifically, there is also a need in the art for a method and system that can passively align a first optical device with a second optical device with high efficiency.
The present invention is generally drawn to a system and method for passively aligning optical devices. More specifically, according to one aspect, the present invention is generally drawn to an external mode size transformer comprising a waveguide having an input section, an output section, and a tapered section disposed between the input and output sections. The input section of the waveguide can comprise a volume having a first cross sectional arm The output section can comprise a volume with a second cross sectional area that is greater than the first cross sectional area. The tapered section can comprise a varying cross sectional area that has cross section substantially equal to the first cross sectional area on one end and a cross section substantially equal to the second cross sectional area on another end.
The input section of the waveguide can be designed to capture a fundamental mode of a first size emanating from a first optical device while the output section can be designed to match a modal profile of a second optical device with a second mode size. In one exemplary embodiment, the second mode size can be larger than the first mode size. For example, the input section can be designed to capture a fundamental mode of a semiconductor laser while the output section of the waveguide can be designed to match a modal profile of a single-mode optical fiber. The tapered section can be designed to gradually transform the modal profile from that of the input section to that of the output section of the waveguide. The cross sections between the input and output sections of the waveguide can vary smoothly throughout the length of the waveguide, similar to a waveguide shaped like a boat hull.
The smoothly varying cross section dimensions of the tapered section of the waveguide is yet one physical property of the waveguide that is not found and cannot be achieved with the conventional art methods. In other words, the smooth and circular or rounded cross sections of the tapered section as well as the input and output sections of the waveguide are not attainable with conventional layering processes. Further, the waveguide can also have smooth or abrupt changes or both in the refractive indexes of the sections that form the waveguide. Such smooth or abrupt changes (or combination thereof) in the refractive indexes of the waveguide sections cannot be achieved with conventional art techniques.
The dimensions of each section of the waveguide as well as the refractive index profile of each section can be controlled during the exemplary manufacturing method according to another aspect of the present invention. The waveguide can have a step index profile that is roughly elliptical with a flat top. The waveguide of the transformer can also comprise glassy materials that can be manufactured with an exemplary ion exchange process according to the manufacturing method of the present invention.
By positioning the external mode size transformer between optical devices, coupling between optical devices is substantially improved compared to that of direct coupling. For example, by positioning the external mode size transformer between a laser and an optical fiber, laser to fiber coupling is significantly improved compared to that of direct coupling between these optical devices. Additionally, coupling efficiency is typically not degraded if either optical device is misaligned with the external mode size transformer by less than one micron. Such a tolerance for misalignment can make the present invention compatible with silicon submount techniques for passive alignment of optical devices. The present invention can be used for fiber coupling of semiconductor optical amplifiers (SOA), and can serve as an optical coupler in various other hybrid-integrated optical devices.
According to another aspect of the present invention, the external mode size transformer can be manufactured with an exemplary ion exchange process. More specifically, the present invention can comprise a manufacturing method that employs multiple masks with predetermined shapes to fabricate the sections of the waveguide within a planar substrate. Using multiple masks can enable the ion diffusion to penetrate to a greater depth in the areas exposed by the masks. During ion diffusion or the ion exchange process, sodium ions within the planar substrate can be replaced with ions of an appropriate metal, such as silver (Ag) or potassium (K) through an electrochemical reaction. The resultant waveguide of the external mode transformer can feature a smoothly varying refractive index cross section throughout the length of the waveguide.
According to yet another aspect of the present invention, the external mode size transformer can be employed in a method for passively aligning optical devices. More specifically, in one exemplary embodiment, the external mode size transformer in addition to a laser diode and an optical fiber can be positioned within a V-groove of a silicon submount. By using the V-groove of the submount and other alignment tools such as cladding strips attached to each optical device that are placed in the V-groove, the external mode size transformer, optical fiber, and laser diode can be passively aligned in an efficient manner.